The present invention relates to a method of feedback-controlling the air-fuel ratio of an internal combustion engine, and more particularly, it relates to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration.
Among methods of feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (referred to hereinafter as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, there is one proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 59-208141, in which the desired air-fuel ratio is changed according to operating conditions of the engine.
In the meanwhile, a system for controlling operating characteristics (valve lift amount, valve opening timing, etc.) of at least one set of a set of intake valves and a set of exhaust valves has been proposed e.g. by Japanese Patent Publication Kokoku) No. 49-33289, in which the operating characteristics of at least one set of the valves is changeable between operating characteristics suitable for a low engine rotational speed region and operating characteristics suitable for a high engine rotational speed, one of the two kinds of operating characteristics being selected according to operating conditions of the engine.
In the above proposed air-fuel ratio control method, the desired air-fuel ratio is changed, specifically, in response to the engine coolant temperature and/or the throttle valve opening. However, it does not take into consideration changes in operating conditions of the engine (in this case, particularly a change in the throttle valve opening) occurring when the gear position of a transmission driven by the engine has been changed, and therefore the proposed method has a problem that the supply air-fuel ratio temporarily undergoes a large deviation from a desired value due to a drastic change in the desired air-fuel ratio occurring when the gear position of the transmission is changed.
Further, when the conventional air-fuel ratio control method is applied to an engine in which the operating characteristics of the intake and/or exhaust valves can be changed over, it is necessary to set the desired air-fuel ratio while taking into consideration a change in the operating characteristics of the valves as well as a change in the gear position of the transmission.